Densification of C-C composites with pitches followed by CVI/CVD

ABSTRACT

A method of manufacturing pitch-based carbon-carbon composite useful as a brake disc, includes (a) providing annular carbon fiber brake disc preform; (b) heat-treating the carbon fiber preform; (c) infiltrating the carbon fiber preform with pitch feedstock by VPI or RTM processing; (d) carbonizing the pitch-infiltrated carbon fiber preform; (e) repeating steps (c) and (d) to achieve a density in the carbon fiber preform of approximately 1.5 g/cc to below 1.7 g/cc; and (f) densifying the preform by CVI/CVD processing to a density higher than 1.7 g/cc. Employing lower cost VPI and/or RTM processing in early pitch densification cycles and using more expensive CVI/CVD processing only in the last densification cycle provides C-C composites in which the pitch-based components resist pullout, resulting in a longer wearing composite.

This application is a continuation of U.S. patent application Ser. No.12/050,499, which was filed on Mar. 18, 2008 and published as U.S.Patent Application Publication No. 2009/0238966 on Sep. 24, 2009. Theentire content of U.S. patent application Ser. No. 12/050,499 isincorporated herein by reference.

TECHNICAL FIELD

This invention relates to the manufacture of pitch-based carbon-carboncomposites useful as friction materials. A preferred embodiment of thepresent invention is an aircraft brake disc made from a carbon fiberpreform that has been densified with pitch.

BACKGROUND

Carbon-carbon composites which have been densified with pitches and usedas friction materials typically exhibit higher wear rates compared withcarbon-carbon composite that have been densified by CVI/CVD processing.The higher wear rate of C-C composites comprised of pitch matricesresults in the friction materials having to be replaced on a morefrequent basis, which has a negative impact on operating costs for theend user.

A series of patent publications by Huang et al. are direct to themanufacture of carbon composites wherein carbon preforms are subjectedto one or two infiltration cycles using pitch or other carbonaceousmaterials to fill voids in the composite. See US 2005/0274581; US2004/0105969; US 2004/0155382; US 2004/0017019; and U.S. Pat. No.6,699,427.

Murdie et al. disclose the formation of carbon composite articles. U.S.Pat. No. 6,323,160. The Murdie disclosure deals with densification ofcarbon preforms by various known techniques, including CVD, CVI, andpitch impregnation followed by carbonization. Murdie et al. teach thatcombinations of these techniques are often used to make a final product.See column 10, lines 34 and following.

U.S. Pat. Nos. 5,061,414 and 5,217,657 (Engle) disclose a method ofmaking carbon-carbon composites from a pitch-impregnated preformfollowed by carbonization, with further pitch-impregnation steps withheat treatments between each impregnation, followed by CVD.

SUMMARY

This invention provides a method of manufacturing a pitch-basedcarbon-carbon composite useful as a brake disc. The method includessequential steps (a) through (f), as follows. Step (a) involvesproviding an annular carbon fiber brake disc preform. Step (b) involvesheat-treating the carbon fiber preform at 1200-2540° C.

Step (c) involves infiltrating the carbon fiber preform with a pitchfeedstock by vacuum pressure infiltration (VPI) or resin transfermolding (RTM) processing. In step (c), the carbon fiber brake discpreform may be densified with synthetic, coal tar, or petroleum derivedmesophase pitch to a density of approximately 1.1-1.5 grams per cubiccentimeter by VPI or RTM processing, or the carbon fiber brake discpreform may be densified with a low cost, synthetic, coal tar, orpetroleum derived, isotropic pitch having a low to medium char yield toa density of approximately 1.1-1.3 grams per cubic centimeter in step(c) by VPI or RTM processing.

Step (d) involves carbonizing the pitch-infiltrated carbon fiber preformat 1200-2200° C. in an inert atmosphere. Step (d) may include anoptional stabilization step prior to carbonization to rigidize the pitchand prevent exudation from the preform during carbonization. Theoptional stabilization step may be oxidative stabilization carried outat a temperature of about 150-250° C. to prevent pitch exudation.Alternatively, pressure and a can may be used to prevent and contain anypitch exudation during carbonization. Step (d) may include an optionalmachining step after carbonization to enhance surface porosity insurface areas of the preform.

“Step (e)” involves the repetition of steps (c) and (d) a sufficientnumber of times to achieve a density in the carbon fiber preform ofapproximately 1.5 g/cc to below 1.7 g/cc. The pitch used in step(e)—that is, one or more repetitions of step (c)—may be a higher costmesophase material (synthetic, coal tar, or petroleum derived) or anisotropic, low to medium char yield pitch (synthetic, coal tar, orpetroleum derived), or step (e)—that is, one or more repetitions of step(c)—may use a combination of these isotropic and mesophase pitches. Insome circumstances, a single cycle of CVI/CVD densification mayoptionally be employed as one of the repetitive densification cyclesencompassed by step (e).

Step (f) involves densifying the preform by chemical vapor deposition(CVD) or chemical vapor infiltration (CVI) to a density higher than 1.7g/cc. The C-C composite densified by CVI/CVD processing in step (f) mayhave: (i.) a rough laminar microstructure and be densified at atemperature of 1275° C., a pressure of 210 Torr, and a C/H ratio of 1/4;(ii.) a smooth laminar microstructure and be densified at a temperatureof 1200° C., a pressure of 630 Torr, and a C/H ratio of 1/4; or (iii.)an isotropic microstructure and be densified at a temperature of 1425°C., a pressure of 630 Torr, and a C/H ratio of 1/4. The densifiedannular carbon-carbon composite brake disc preform resulting from step(f) may be subjected to a final heat-treatment at 1200-2540° C.,typically at a temperature between about 1400 and 2540° C. in an inertnitrogen atmosphere or in a vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart depicting the sequential processing steps of thepresent invention.

DETAILED DESCRIPTION

The present invention describes a precise sequence of processing stepsused to manufacture C-C composite friction materials with improvedfriction and wear performance. In accordance with the present invention,the addition of a final CVI/CVD densification step following multiplecycles of pitch densification via VPI or RTM reduces the wear rate ofthe friction material. The final CVD/CVI step serves to improve thebinding of the pitch matrix throughout the composite, and helps toreduce the amount of fiber and matrix pull-out during the frictionprocess, resulting in a reduction to the wear rate of the composite.

This invention employs the following sequential steps:

(a) providing an annular nonwoven or chopped carbon fiber brake discpreform;

(b) heat-treating the carbon fiber preform at 1200-2540° C.;

(c) infiltrating the carbon fiber preform with a low-to-medium charyield isotropic pitch (coal tar, petroleum, or synthetic) or a high charyield mesophase pitch by vacuum pressure infiltration (VPI) or resintransfer molding (RTM) processing;

(d1) an optional stabilization step prior to carbonization to rigidizethe pitch and prevent exudation from the preform during carbonization,which step comprises heating the preform in air at 150-250° C.;

(d2) carbonizing the pitch-infiltrated carbon fiber preform at1200-2200° C. in an inert atmosphere;

(d3) an optional machining step after carbonization to grind thesurfaces of the preform (using standard grinding equipment), therebyopening surface porosity in the preform;

(e) repeating steps (c) and (d) to achieve a density in the carbon fiberpreform of approximately 1.5 g/cc to below 1.7 g/cc; and

(f) densifying the preform by chemical vapor deposition (CVD) orchemical vapor infiltration (CVI) to a density higher than 1.70 g/cc.

Subsequent to the above steps, the dense annular carbon-carbon compositebrake disc preform resulting from step (f) may be subjected to a finalheat-treatment at 1200-2540° C. It will also normally be subjected toadditional conventional processing steps, including final machining andtreatment with antioxidant solutions.

The present invention makes use of processing modules which are known inthemselves. The advantages provided by the present invention lie in theselection and ordering of known processing modules to improve thefriction and wear performance of the C-C composite brake discs preparedin accordance with this invention as compared with standardpitch-infiltrated brake discs. This invention reduces the wear rates ofC-C composite friction materials by applying a final CVI/CVD cyclefollowing multiple pitch infiltrations. The final CVI/CVD step alsoimproves the strength and oxidation resistance by improving bonding ofthe matrix throughout the composite. The present invention likewiseimproves the economics of disc manufacture. Various “modules” that maybe used in accordance with the present invention are summarized below.

Heat Treatment.

Intermediate and/or final heat treatment of the preforms is usuallyapplied to modify the crystal structure and order of the carbon. Heattreatment is employed to modify the mechanical, thermal, and chemicalproperties of the carbon in the preform. Heat treatment of the preformsmay be conducted in the range of 1600° to 2800° C. The effect of such atreatment on graphitizable materials is well known. Higher temperaturesincrease the degree of order in the material, as measured by suchanalytical techniques as X-ray diffraction or Raman spectroscopy. Highertemperatures also increase the thermal conductivity of the carbon in theproducts, as well as the elastic modulus.

VPI.

Vacuum Pressure Infiltration (“VPI”) is a well known method forimpregnating a resin or pitch into a preform. The preform is heatedunder inert conditions to well above the melting point of theimpregnating pitch. Then, the gas in the pores is removed by evacuatingthe preform. Finally, molten pitch is allowed to infiltrate the part, asthe overall pressure is returned to one atmosphere or above. In the VPIprocess a volume of resin or pitch is melted in one vessel while theporous preforms are contained in a second vessel under vacuum. Themolten resin or pitch is transferred from vessel one into the porouspreforms contained in the second vessel using a combination of vacuumand pressure. The VPI process typically employs resin and pitches whichpossess low to medium viscosity. Such pitches provide lower carbonyields than do mesophase pitches. Accordingly, at least one additionalcycle of pitch infiltration of low or medium char-yield pitch (with VPIor RTM processing) is usually required to achieve a final density of 1.7g/cc or higher.

Carbonization

The carbonization process is generally well known to those skilled inthe art. The CVD/resin/pitch-infiltrated fiber preforms are heated in aretort under inert or reducing conditions to remove the non-carbonconstituents (hydrogen, nitrogen, oxygen, etc.) from the fibers andmatrix carbons. This process may be performed, for instance, by buryingthe foam preforms in a bed of activated carbon, enclosed in a superalloyretort with a sand seal. Carbonization of the infiltrated pitch can becarried out either in a furnace, a hot isostatic press, an autoclave, orin a uniaxial hot press. In each of these techniques, the impregnatedpart is heated to the range of 600° to about 1000° C., while maintainingan inert atmosphere in the pressure range of 1 to 1000 atmospheres. Inone approach, for instance, the retort is purged gently with nitrogenfor approximately 1 hour, then it is heated to 900° C. in 10-20 hours,and thence to 1050° C. in 1-2 hours. The retort is held at 1050° C. for3-6 hours, then allowed to cool overnight. Carbonization can be carriedout up to 1800° C. The higher the pressure, the higher the carbon yieldachieved, although the biggest gains in carbon yield are achieved atmoderate pressures up to 5000 psi.

Machining the Surfaces of the Preform.

Standard machining processes, well know to persons skilled in the art ofmanufacturing carbon-carbon composite brake discs, are used in themanufacture of the carbon-carbon composite friction discs provided bythe present invention. Between densification processing steps, thesurfaces of the annular discs are ground down to expose porosity in thesurfaces. Once the final density is achieved, the annular discs areground to their final thickness using standard grinding equipment toprovide parallel flat surfaces, and then the inside diameter and outsidediameter regions are machined, typically using a CNC (computer numericalcontrol) Mill to provide the final brake disc geometry, including suchfeatures as rivet holes and drive lugs.

CVD/CVI.

Chemical vapor deposition (CVD) of carbon is also known as chemicalvapor infiltration (CVI). In a CVD/CVI process, carbonized, andoptionally heat treated, preforms are heated in a retort under the coverof inert gas, typically at a pressure below 100 torr. When the partsreach a temperature of 900° to 1200° C., the inert gas is replaced witha carbon-bearing gas such as methane, ethane, propane, butane,propylene, or acetylene, or combinations of these gases. When thehydrocarbon gas mixture flows around and through the porous structures,a complex set of dehydrogenation, condensation, and polymerizationreactions occur, thereby depositing the carbon atoms within the interiorand onto the surface of the porous structures. Over time, as more andmore of the carbon atoms are deposited onto the structures, the porousstructures become more dense. This process is sometimes referred to asdensification, because the open spaces in the porous structures areeventually filled with a carbon matrix until generally solid carbonparts are formed. Depending upon the pressure, temperature, and gascomposition, the crystallographic structure and order of the depositedcarbon can be controlled, yielding anything from an isotropic carbon toa highly anisotropic, ordered carbon. US 2006/0046059 A1 (Arico et al.),the disclosure of which is incorporated herein by reference, provides anoverview of CVD/CVI processing.

EXAMPLES

With the present invention, there are several process options availableincluding variations in: the type of pitch used (isotropic coal tar orpetroleum vs. mesophase pitch); the method of infiltration (VPI vs.RTM); the carbonization temperature; and the final heat treatmenttemperature, etc. A preferred embodiment of this invention wouldinclude: carbonization of the carbon fiber preform to 1800° C.;densifying the preform first with AR synthetic pitch by RTM to providesa uniform microstructure at the fiber-matrix interfaces throughout thethickness of the composite; stabilizing the mesophase pitch by heatingin air between 150 and 190° C.; performing subsequent pitchdensification cycles with isotropic coal tar pitch by VPI to provide aneconomical method to achieve a final density greater than 1.7 g/cc;final CVD densification to bond the pitch matrix into the C-C Composite;and final heat treatment at 2000° C.

The Table below shows some of the options that can be used to provide alow cost densification process with improved friction and wearperformance in accordance with the present invention.

Carbon- Stabili- Carbon- Carbon- Preform ization 1^(st) Dens. zationization 2^(nd) Dens. ization 3^(rd) Dens. Non- 1600° C. Mesophase 170°C. 2000° C. Isotropic coal 1600° C. CVI-Rough woven pitch/RTM tarpitch/RTM Laminar Random 2000° C. Mesophase N/A 1800° C. Isotropic coal2200° C. CVI Fiber pitch/VPI tar pitch/VPI Isotropic Non 1200° C.Isotropic 180° C. 1600° C. Isotropic coal 1600° C. CVI smooth wovenpitch/RTM tar pitch/VPI laminar Random 1600° C. Mesophase 160° C. 1400°C. Isotropic coal 1600° C. CVI-Rough fiber pitch/VPI tar pitch/RTMLaminar Non- 2000° C. Mesophase N/A 2200° C. Isotropic coal 1800° C. CVIsmooth woven pitch/VPI tar pitch/RTM laminar Random 1400° C. Mesophase190° C. 2540 C. Isotropic coal 1800° C. CVI fiber pitch/VPI tarpitch/VPI Isotropic Non- 2540° C. Mesophase N/A 1200 C. Isotropic coal1600° C. CVI Rough woven pitch/VPI tar pitch/RTM Laminar

INDUSTRIAL APPLICABILITY

In terms of manufacturing economics, the hybrid composite conceptembodied in the present invention enables the use of low cost pitchmaterials and processes to be combined with only one cycle of CVI/CVDand its associated high cost of capitalization to provide C-C compositefriction materials with improved friction and wear performance at lowercost.

1. A method of manufacturing a pitch-based carbon-carbon compositeuseful as a brake disc, the method comprising the following sequentialsteps: (a) heat-treating an annular carbon fiber preform comprising anonwoven fabric at 1200° C.-2540° C.; (b) densifying the preform byinfiltrating the carbon fiber preform with a pitch feedstock by vacuumpressure infiltration (VPI) or resin transfer molding (RTM) processing;(c) carbonizing the pitch-infiltrated carbon fiber preform at 1200°C.-2200° C. in an inert atmosphere; (d) repeating steps (b) and (c) toachieve a density in the carbon fiber preform of approximately 1.5 gramsper cubic centimeter (g/cc) to below 1.7 g/cc; and (e) densifying thepreform by a single cycle of chemical vapor deposition (CVD) or chemicalvapor infiltration (CVI) to provide a C-C composite which: (i.) has arough laminar microstructure and is densified at a temperature of 1275°C., a pressure of 210 Torr, and a C/H ratio of 1/4; (ii.) has a smoothlaminar microstructure and is densified at a temperature of 1200° C., apressure of 630 Ton, and a C/H ratio of 1/4; or (iii.) has an isotropicmicrostructure and is densified at a temperature of 1425° C., a pressureof 630 Ton, and a C/H ratio of 1/4, wherein said C-C composite has adensity higher than 1.7 g/cc.
 2. The method of claim 1, furthercomprising, prior to carbonizing the pitch-filtrated carbon fiberpreform, stabilizing the preform to rigidize the pitch and preventexudation from the preform during carbonization.
 3. The method of claim2, stabilizing the preform comprises stabilizing the preform viaoxidative stabilization carried out at a temperature of about 150°C.-250° C. to prevent pitch exudation.
 4. The method of claim 1, furthercomprising, after carbonizing the pitch-filtrated carbon fiber preform,machining the preform to enhance surface porosity in surface areas ofthe preform.
 5. The method of claim 1, further comprising subjecting thecarbon-carbon composite brake disc preform resulting from step (e) to afinal heat-treatment at 1200° C.-2540° C.
 6. The method of claim 5,wherein a final heat-treatment step is carried out at a temperaturebetween about 1400° C. and 2540° C. in an inert nitrogen atmosphere orin a vacuum.
 7. The method of claim 1, wherein densifying the preform byinfiltrating the carbon fiber preform with a pitch feedstock by vacuumpressure infiltration (VPI) or resin transfer molding (RTM) processingin step (b) comprises densifying the preform with mesophase pitch to adensity of approximately 1.1 g/cc-1.5 g/cc in step (b) by VPI or RTMprocessing.
 8. The method of claim 7, where the mesophase pitch used instep (b) is synthetic, coal tar, or petroleum derived.
 9. The method ofclaim 1, wherein densifying the preform by infiltrating the carbon fiberpreform with a pitch feedstock by vacuum pressure infiltration (VPI) orresin transfer molding (RTM) processing in step (b) comprises densifyingthe preform with an isotropic pitch having a low to medium char yield toa density of approximately 1.1 grams per cubic centimeter (g/cc)-1.3g/cc in step (b) by VPI or RTM processing.
 10. The method of claim 9,where the isotropic pitch used in step (b) is synthetic, coal tar, orpetroleum derived.
 11. The method of claim 1, where the pitch used instep (d) is a mesophase material.
 12. The method of claim 11, whereinthe mesophase material is synthetic, coal tar, or petroleum derived. 13.The method of claim 1, where the pitch used in step (d) is an isotropic,low to medium char yield pitch.
 14. The method of claim 13, wherein theisotropic, low to medium char yield pitch is synthetic, coal tar, orpetroleum derived.
 15. The method of claim 1, where the pitch used instep (d) is a combination of isotropic and mesophase pitches.
 16. Themethod of claim 15, wherein the pitch is synthetic, coal tar, orpetroleum derived.
 17. The method of claim 1, wherein pressure and a canare used to prevent and contain any pitch exudation duringcarbonization.